When a pump receives fluid under pressure, it may use this fluid under pressure to drive the pump. One example is a pressure intensifier described in Solomon U.S. Pat. No. 4,995,793 which increases the pressure of the feedwater by some desired ratio. Another pump of this type is an injector pump which reduces the pressure of the feedwater, and a pump of this type is shown in Solomon U.S. Pat. No. 5,009,777. In both cases, the pressure of the feedwater is the source of energy for driving the pump and so no electric motor or wiring is required. This makes pumps of this type desirable for many applications including reverse osmosis water systems which are used for removing impurities from water, such as drinking water.
Pumps of this type are commonly reciprocating and have a piston which moves on pumping and reset strokes. One important consideration in a pump of this kind is to assure that the piston does not "hang up" or stall at the end of a stroke. In other words, it is important that the piston properly switch from each pumping stroke to a reset stroke and from each reset stroke to the next pumping stroke.
Another consideration is charging of the outlet chamber with feedwater during a reset stroke so that this water can be pumped out of the outlet chamber during the next pumping stroke. On way to accomplish this is with a check valve as shown in Solomon U.S. Pat. No. 4,995,793. Another way to accomplish this is by using a valve external to the pump or by a spool valve as shown by way of example in Solomon U.S. Pat. No. 5,009,777.